The present invention relates to a system which is capable of generating an EMF equivalent to that generated by a thermocouple of a selected type at a selected temperature.
A thermocouple consists of a pair of wires, each made of a different type of metal, and having one set of ends joined together to form a sensing junction. The other set of ends of the wires terminate so that they are at the same, known temperature to form a reference junction. Wires are connected between the reference junction and a load such as an indicating meter or the input of other read-out or signal conditioning equipment. The wires which connect the reference junction and the load may be made of copper or of another type of metal different from the metals joined at the sensing junction.
Whenever the sensing junction and the reference junction of a thermocouple are at different temperatures, a current flows through the circuit due to the thermoelectric effect. The magnitude of the electromotive force (EMF) which causes the flow of current through the circuit is dependent on the type of metals which are joined to form the sensing junction, as well as on the temperatures of the sensing junction and the reference junction. Metals which are commonly used in thermocouples are CHROMEL (Trademark) and ALUMEL (Trademark), Constantan, Copper, Iron, Platinum, an alloy of Platinum and Rhodium, Tungsten, Tungsten-Rhenium alloys, Nickel, and Ferrous Nickel alloys. There are hundreds of types of thermocouples in use.
The characteristics of thermocouples made of different combinations of metals, such as their temperature versus EMF curves, their accuracy tolerances, and their wire insulation color codes, have been standardized by American National Standards Institute Standard C96.1. The ANSI Standard favors the use of typeletter designations in lieu of names of the two metals used in a particular thermocouple. For example, the following are typical thermocouple types: E (CHROMEL-/Constantan), J (Iron/Constantan), K (CHROMEL/ALUMEL) and T (Copper/Constantan). In National Bureau of Standards (NBS) Monograph 125, polynomials are specified which define the temperature versus EMF curves of the various standard thermocouple types.
Many temperature measuring devices, temperature control devices and other related devices are designed to be connected to thermocouples. Such devices need to be calibrated. Therefore, it is desirable to have a system which is capable of generating an EMF equivalent to that generated by a thermocouple of a selected type operated at a selected temperature. Furthermore, there exists a need for a system that will enable precise calibration of conventional DC measuring devices and thermocouples.
U.S. Pat. No. 3,504,522 of Jasik et al discloses a thermocouple system simulator including an operational amplifier having a DC current voltage source and a resistance bridge disposed in the feedback loop thereof. The resistance bridge includes in one leg thereof a device whose resistance varies with temperature. The feedback loop further includes a thermocouple.
A Model 1100 Thermocouple Simulator/Calibrator is commercially manufactured by Ectron Corporation of San Diego, Calif., the assignee of the subject application. Its output voltage is a succession of straight line segments which approximate type E, J, K or T thermocouple curves. The output voltage is generated by two multiplying DACs. Two of the DAC inputs are selected from ROM memory. A third DAC input is obtained directly from the setting of lever switches on the front panel of the device. The Model 1100 requires an oven assembly for maintaining reference junctions at a constant temperature. It further utilizes output terminals which are made of the actual metals and alloys of the various thermocouple types which are simulated.